A method is known from the prior art for manufacturing a steel reinforcing element for a tire, for example a steel thread.
The manufacturing method makes it possible to manufacture, from a wire, referred to as wire stock, having an initial diameter between 4.5 and 7.5 mm, a wire that can be used for the manufacture of a tire, in particular a wire that can be used for reinforcing plies of the tire, the wire having a diameter of between 0.08 mm and 0.50 mm.
Firstly, the wire stock of predominantly pearlitic microstructure is drawn, for example in a dry environment, so as to reduce its initial diameter to an intermediate diameter, for example equal to 1.3 mm. At the end of this drawing step, the steel of the wire has a microstructure comprising several mixed phases.
Next, the wire of intermediate diameter is heat treated so as to modify the microstructure of the steel. In this instance, the predominately pearlitic microstructure of the steel is regenerated.
After having coated the wire of intermediate diameter with a metal layer, the coated wire of intermediate diameter is drawn, for example in a wet environment, so as to reduce its diameter to a final diameter, for example equal to 0.20 mm.
A method for the heat treatment of the wire of intermediate diameter is known from U.S. Pat. No. 4,767,472 that comprises three steps and is carried out by means of a heat treatment facility.
The heat treatment facility comprises, in the run direction of the wire, upstream means for storing the untreated wire, for example upstream reels, a heating device, a cooling device, and downstream means for storing the treated wire, for example downstream reels.
During a first step, the temperature of the wire is increased above the austenitizing temperature of the steel in order to obtain a predominantly austenitic microstructure. For this purpose, the facility comprises a device for heating the wire comprising a gas-fired furnace.
Then, in a second step carried out downstream of the gas-fired furnace, the temperature of the wire in the austenite stability range is reduced For this purpose, the facility comprises a cooling device comprising a water bath. The bath comprises substantially pure liquid water at a temperature above 80° C. through which the wire is made to run.
In a third step, carried out downstream of the bath, the temperature of the wire is left to drop in the ambient air or else in a thermally insulated device. During this exposure to the ambient air, the predominantly austenitic microstructure is transformed to a predominately pearlitic microstructure or else the transformation, pre-initiated in the bath, is continued by passing through the pearlite transformation range.
However, the method of U.S. Pat. No. 4,767,472 requires a precise control of the cooling rates in the various ranges.
Indeed, it is necessary to very precisely control the rate of temperature reduction in the austenite range in order to return to the transformation ranges at a relatively precise predetermined temperature. By using too high a rate of reduction, there is a risk of carrying out the transformation by passing through martensite and/or bainite transformation ranges which would result in a relatively brittle wire that would therefore be unusable during a subsequent drawing step. By using too low a rate of reduction, there is a risk of obtaining too large an amount of ferrite which would result in a wire having low mechanical properties, in particular tensile strength.
Moreover, it is necessary to pass through the transformation ranges without a large change in the temperature of the wire, taking into account in particular the recalescence. Otherwise, structural heterogeneities are created that result in a loss of mechanical strength of the steel and in a poorer processability, in particular a lower work hardening capacity of the steel.